1. Field of the Invention
The invention relates to an exhaust gas recirculation system for an internal combustion engine, and a method for controlling the same.
2. Description of the Related Art
An exhaust gas recirculation system that recirculates a portion of the exhaust gas back to an intake system of an internal combustion engine has been suggested in order to reduce the amount of nitrogen oxide (NOx) that is generated when fuel is burned in the internal combustion engine.
In recent years, a technology that makes it possible to perform EGR in a broader operating range of an internal combustion engine has been suggested. As described in, for example, Japanese Patent Application Publication No. 2005-076456 (JP-A-2005-076456), an exhaust gas recirculation system according to the technology includes a high-pressure EGR unit and a low-pressure EGR unit, and recirculates a portion of the exhaust gas back to the internal combustion engine while changing the EGR unit used for exhaust gas recirculation between the high-pressure EGR unit and the low-pressure EGR unit or using both the high-pressure EGR unit and the low-pressure EGR unit in combination, based on the operation mode of the internal combustion engine. The high-pressure EGR unit recirculates a portion of the exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between an exhaust passage, at a portion upstream of a turbine of a turbocharger, and an intake passage, at a portion downstream of a compressor of the turbocharger. The low-pressure EGR unit recirculates a portion of the exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor. Japanese Patent Application Publication No. 2004-027956 (JP-A-2004-027956) and Japanese Patent Application Publication No. 09-151761 (JP-A-09-151761) also describe technologies related to exhaust gas recirculation.
Because a low-pressure EGR passage is usually long, the amount of exhaust gas in the low-pressure EGR passage is relatively large. Accordingly, even when the operation mode of an internal combustion engine changes, it takes a relatively long time for the exhaust gas, which is discharged from the internal combustion engine in the pre-change operation mode and which still remains in the low-pressure EGR passage, to be entirely taken in cylinders of the internal combustion engine. During this time period, the exhaust gas, which has characteristics (the concentrations of the components, the temperature, etc, of the exhaust gas) different from those of the exhaust gas that is discharged from the internal combustion engine in the post-change operation mode is taken in the internal combustion engine. Accordingly, the oxygen concentration in the intake air (or the EGR rate) may deviate from the target oxygen concentration in the intake air (or the target EGR rate), resulting in an increase in the toxic substance contained in the exhaust gas, incomplete combustion, etc.